This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-301472, filed Sep. 29, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a disk apparatus for performing recording/reproduction on a disk having information recording tracks, more specifically, it relates to a method of access control of a disk apparatus that performs positioning on a target track with use of two actuators: coarse and precise actuators.
In the field of optical disks, represented by DVDS, a disk having a diameter of 120 mm and a recording capacity of 4.7 GB has been brought into practical use in recent years. It is also expected that a disk having a recording capacity 3-4 times larger than that of this disk will be developed for practical use several years later. In order to cope with the increase in the recording density in the disk due to the increase in the recording capacity, more specifically, with the reduction of a track pitch, the servo precision having more precise positioning performance is now required for a disk recording/reproducing drive.
In fact, the highly precise positioning technique is essential for an optical disk apparatus to realize positioning of an optical spot in the direction of the diameter of a disk in the order of several nm on information recording tracks each having a pitch of 1 xcexcm or less. On the other hand, the access speed is also an important factor of the performance of the disk drive. A disk apparatus thus needs an access control technique capable of positioning, satisfying both the demands of high-speed and high precision.
Generally, in order to cover the diameter of a disk and realize highly precise positioning, the positioning of the optical spot is performed with two actuators, i.e., coarse and precise actuators. Access control, using these two actuators, in order to realize the access with high-speed and high precision, will now be described.
These two actuators, however, have some limitations.
The limitation of the precise actuator is that in displacement amount. In accordance with the increase of the optical disk in density, the requirements of the specification of optical elements used for the optical disk apparatus are becoming stricter. For example, the shift of an objective lens from the optical axis is required to be 50 xcexcm or less for a 4.7 GB DVD disk. If the shift is larger, the optical aberration becomes larger, and the recording/reproduction of information or detection of a positioning signal is adversely affected. A biaxial actuator is used as a precise actuator for positioning an objective lens, and has a limitation in the displacement amount.
The limitation of the coarse actuator is poor positioning precision. The coarse actuator is intended to coarsely position the objective lens in the direction of the radius of a disk. To cope with the requirement for reduction of manufacturing cost, the coarse actuator is formed of low-cost components: a sliding bearing and a low-cost positioning motor. Similarly, a driving force transmission mechanism is subject to many non-linear factors, such as friction and backlash. The positioning precision of the coarse actuator is as poor as 100 xcexcm.
It is now required for the optical disk apparatus to perform the positioning satisfying both high-speed and high precision with use of the two actuators without large displacement of the precise actuator. Some access control methods have been proposed to overcome the above-mentioned limitations by suitably making the two actuators cooperate with each other.
The conventional access control methods by the cooperation of the precise and coarse actuators will be described below in conjunction with the block diagrams of FIGS. 2 and 8. The constitution shown in these diagrams comprises a feed motor (a coarse actuator) 107 for moving an optical head 103, and an objective lens actuator (a precise actuator) 106 for positioning only an objective lens 102, in order to position the optical spot on a disk 101.
The access control according to the above-mentioned apparatus is performed with use of reflected light from the disk 101 after canceling the tracking control shown in FIG. 8.
In the tracking state, a track traversing signal generated when the optical spot traverses a track on the disk 101 is detected by a light detector 105 and a comparison processing circuit 108. The track traversing signal is input into a precise positioning mechanism controlling compensator 402 and then input into a precise positioning mechanism 106 via an amplifier 13 and a precise positioning mechanism driving circuit 15. The precise positioning mechanism 106 is controlled so as to move the optical spot to the same track. The track traversing signal is also supplied to a coarse positioning mechanism controlling compensator 403 and then input into a coarse positioning mechanism 107 via an amplifier 16 and a coarse positioning mechanism driving circuit 19. Similarly to the precise positioning mechanism 106, the coarse positioning mechanism 107 is controlled so as to move the optical spot to the same track.
Simultaneously to the staring the access, the tracking state is switched to the control state controlled by the control system constituted as shown in the block diagram of FIG. 2. In the access control by the control system, the light detector 105 detects the track traversing signal (the tracking error signal). A counter 110 counts the number of traversed tracks on the basis of the track traversing signal. A reference speed generator 111 generates a target speed (a reference speed) based on the number of the remaining tracks.
On the other hand, a speed detector 114 binarizes the track traversing signal to generate a track count pulse, then divides a track pitch by an interval of the track count pulses to obtain a relative moving speed on the tracks. A gain compensator 113 compensates the difference between the relative moving speed and the reference speed by amplification to output a drive signal to the feed motor 107 as the coarse actuator.
According to the conventional method, the access control by the speed control is performed at first, wherein the difference between the relative moving speed and the reference speed is fed back to the feed motor 107 until the detected relative moving speed equals the reference speed. It is found that the detected relative moving speed equals the reference speed when the output of the gain compensator 113, which indicates the difference thereof, becomes 0. When the detected relative moving speed equals the reference speed, a control circuit 119 switches Sw1 and Sw2.
After switching Sw1 and Sw2, the objective lens actuator 106 is controlled by the difference between the relative moving speed and the reference speed, and the feed motor 107 is controlled by the output from a displacement sensor 104 for detecting the amount of shift of the objective lens 102 from the optical axis.
The objective lens actuator 106 is subjected to the speed control, and the feed motor 107 is controlled so as to move in accordance with the displacement of the objective lens 102. Such a control system is constituted to perform acceleration with the maximum capacity in high-speed access in order to shorten the access time, and in low-speed access, to control the coarse and precise actuators so as to cooperate for attaining the precise access.
The above-mentioned constitution, however, essentially needs an expensive sensor for actually detecting the relative displacement of the objective lens, and thus the reduction in the manufacturing cost cannot be attained. In addition, the feed motor and the driving force transmission mechanism is subject to non-linear factors and delay factors such as friction, as described above, and it thus may frequently occur that the displacement of the actuator may not actually attained with use of fine output from the sensor. The access speed may be decreased, at the same time. Further, the non-linear elements of the driving force transmission mechanism may adversely affect the start of access at which the tracking state is transferred to the access state, with the result that some problems occur in which the feed motor does not displace at the start of access.
In general, the objective lens actuator has a higher drive sensitivity and higher response frequency than those of the feed motor. Therefore, in the case where the feed motor is controlled to move in accordance with the movement of the objective lens actuator, it cannot be prevented that the feed motor displaces with a low speed after a considerable period of time during which the objective lens actuator has made a large displacement. The large displacement of the objective lens actuator results in large shifts of the objective lens from the optical axis, and a deterioration of the optical signal.
With the conventional access control method, the feed motor is controlled to move in accordance with the movement of the objective lens actuator in consideration of the above-mentioned limitations of the actuators. However, the response speed of the feed motor is so low that a large shift of the objective lens will inevitably occur.
To sum up, it is difficult with the conventional constitution of the optical disk apparatus to overcome the limitations of the actuators. There is concern that this problem will be more serious in future optical disk apparatus used for higher density disks.
The present invention is developed to solve the above-mentioned problems, and intended to provide a disk apparatus capable of precise and stabilized access and seek control with high speed without increasing the manufacturing cost.
According to the aspects of the present invention, a disk apparatus comprising: information recording/reproducing mechanism which records/reproduces information on a disk having a plurality of information tracks; a precise positioning mechanism which finely displaces the information recording/reproducing mechanism in a radial direction of the disk; a coarse positioning mechanism which positionally displaces the information recording/reproducing mechanism in an entire information recording region in the radial direction of the disk; a speed detector which detects a moving speed of the information recording/reproducing mechanism in the radial direction of the disk in accordance with a signal reproduced by the information recording/reproducing mechanism; a displacement calculator which detects a moving distance of the information recording/reproducing mechanism in the radial direction of the disk in accordance with a signal reproduced by the information recording/reproducing mechanism; a reference speed generator which generates a reference speed used for controlling the drive of the precise positioning mechanism in accordance with the displacement calculated by the displacement calculator; a drive signal generator which generates a drive signal for driving the coarse positioning mechanism in accordance with the displacement calculated by the displacement calculator; a coarse positioning mechanism driving circuit which controls the drive of the coarse positioning mechanism in accordance with the drive signal generated by the drive signal generator; and a precise positioning mechanism driving circuit which controls the drive of the precise positioning mechanism in accordance with a difference between the moving speed detected by the speed detector and the reference speed generated by the reference speed generator.